This invention relates to a method and apparatus for imaging acoustic fields in high-frequency acoustic resonators. More particularly, the invention is directed to a scanning RF mode microscope system that detects and monitors vibration of high-frequency resonator devices that vibrate in the frequency range of approximately 1 MHz to 20 GHz. The system then maps vibration modes of such devices to obtain quantitative measurements of the piezoelectric properties of the thin-film materials.
While the invention is particularly directed to the art of imaging acoustic fields in acoustic resonators, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, the invention may be used in any application where detection of high frequency movement of small tightly packaged devices is desired.
By way of background, bulk piezoelectric resonators are utilized as frequency references and filters in the 1 MHZ to 400 MHZ frequency range. A common example is the quartz crystal oscillator used in watches. At these low frequencies, these devices are relatively large (several millimeters). Bulk oscillations of such large devices is achieved, resulting in typical mode shapes ranging in size from hundreds of micrometers to several millimeters.
At higher frequencies, (0.9 GHz to 10 GHz) resonators to be used in wireless communication systems are produced by launching Surface Acoustic Waves (SAW) on bulk crystalline piezoelectrics, or by the manufacture of bulk acoustic wave thin film resonators (TFR). In these latter devices, a piezoelectric film having a thickness of approximately a micrometer is used.
While substantial progress has been made in high-frequency filter design and fabrication, the quality factor Q of such devices is undesirably limited to less than 1000, and consequently the power handling is limited by insertion loss to .about.33 dBm. To understand this limitation, i.e. whether it stems from fundamental materials properties of the piezoelectric thin films, from device design or from basic physical behavior of the device, it is important to implement techniques that allow for a direct study of the mechanical properties of the device in operation.
Previous state of the art techniques, developed mainly for imaging the mode of vibration of quartz oscillators, are not capable of detecting the vibration modes of high frequency resonators. The techniques used for mapping low-frequency quartz oscillators rely on the large volume of those samples, and the relatively long-range vibration patterns. For example, the X-ray imaging technique used in quartz oscillators requires a sample volume of at least a cubic millimeter, and resolves vibration patterns of more than one millimeter. On the other hand, in high-frequency acoustic resonators, the vibration of the device is produced over a few cubic micrometers.
More specifically, "Observation of resonant vibrations and defect structure in single crystals by x-ray diffraction topography", W. J. Spencer, in "Physical Acoustics, Principles and Methods", edited by Warren P. Mason, volume V, pages 111-161, Academic Press (1968), gives a detailed presentation of the many X-ray based methods to observe vibration in quartz oscillators. In general, these methods require relatively large oscillators, since as in most scattering techniques, the sensitivity of x-ray diffraction methods improves with large sample volume. Typically, the quartz oscillators studied in this reference had lateral dimensions of 15 mm, and thickness of several mm. The requirement of large sample volume renders this technique inadequate for the imaging of vibration modes of thin film resonators.
In addition, "Piezoelectric measurements with atomic force microscopy, J. A. Christman, R. R. Woolcott, Jr., A. I. Kingon, and R. J. Nemanich, Applied Physics Letters, volume 73, pages 3851-3853 (1998), presents measurements of the piezoelectric coefficient of various thin film materials. These measurements are performed by using an AFM-based technique. Christman et al., however, do not modulate the amplitude of the drive voltage and do not use a phase-locked loop operating at the modulating frequency. As a result, the Christman et al. method is limited to the low frequencies allowed by their cantilever arm. All the results reported by Christman et al. were obtained at 1 kHz. This limitation renders this technique inadequate for the measurement of piezoelectric properties of materials at high frequencies and for the observation of the vibration modes of high frequency devices.
"High resolution visualization of acoustic wave fields within surface acoustic wave devices", T. Hesjedal, E. Chilla, and H.-J. Frolich, Applied Physics Letters, volume 70, pages 1372-1374, presents an AFM-based technique to image the vibration of surface acoustic waves devices. However, the highest frequency of operation demonstrated by Hesjedal et al. is 602.7 MHz, with no claims about the operability of their system at higher frequencies. In addition, Hesjedal et al. claim that the performance of their set-up is intrinsically non-linear. This nonlinear behavior prevents any quantitative measurement.
The present invention contemplates a new method and apparatus for imaging acoustic fields in high-frequency acoustic resonators that resolve the above-referenced difficulties (and others) and achieve desired operation.